Large scale synthesis of graphene by copper-based chemical vapor deposition (CVD) methods is considered a potential route for graphene commercialization. Unfortunately, CVD graphene films consist of many small domains of graphene crystals that are stitched together. These resulting graphene has defective domain boundaries, and the average domain size remains in the 10-100 micrometer size range.
The properties of polycrystalline materials are often dominated by the atomic structure of their domain boundaries rather the single crystal domains themselves. Such polycrystalline graphene sheets are inferior to single domain graphene because grain boundaries can add resistance and random variation in uniformity. The multi-domain structure of polycrystalline graphene severely degrades its electrical and thermal conductivity as well as its mechanical and chemical properties. When such films are strained, grain boundaries increase the electrical resistance and make such films brittle and porous. These properties severely limit their potential for use in flexible touch panels and/or as gas barrier films (e.g., flexible gas barrier films).
Current CVD processes for manufacturing graphene are inadequate because they produce graphene with the deficiencies described above. For example, CVD techniques can yield random crystallization and growth of graphene. In other words, the graphene crystal size and location is uncontrolled. Additionally, CVD processes can be very slow (e.g., approximately 1 day for a 1 cm2 crystal of graphene). Another challenge for CVD techniques is the integration of high throughput manufacturing techniques with the high temperature requirement of the CVD chamber. Typically, the entire CVD chamber is heated to a high temperature (up to approximately 1,000° C.). The flammable gas environment can damage the equipment if it comes into contact with air during sample loading or unloading, which leads to slow processing due to extensive environment control systems (high vacuum pumps etc.). Additionally, substrates need to be cooled slowly since non-uniform temperature drops over a large substrate can results in a warped substrate. Since cooling is faster at the edges of the substrate than the center, an environment having too large a temperature difference can yield non-uniform temperatures of the substrate. Therefore, the substrate cannot be immediately moved into a cool environment, which slows down the production rate and can be a limiting factor for commercial-scale production. Additionally, the substrate cannot be rolled upon itself or stacked upon itself within the growth system without damaging the graphene.
Accordingly, there is a need for improved processes for making graphene that is more crystalline and has more predictable crystalline boundaries, thereby enabling commercial scale production.